<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="catalog_product_save_after">
        <observer name="test_name" instance="YOUR_NAMESPACEYOUR_MODULENAMEObserverProductsaveafter" />
    </event>
</config>

<?php

namespace YOURNAMESPACEYOURMODULENAMEObserver;

use MagentoFrameworkEventObserverInterface;

class Productsaveafter implements ObserverInterface
{
    protected $_productloader;


    public function __construct(
        MagentoCatalogModelProductFactory $_productloader

    ) {
        $this->_productloader = $_productloader;
    }

    public function getLoadProduct($id)
    {
        return $this->_productloader->create()->load($id);
    }

    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $value = 'Your value';
        $_product = $observer->getProduct();  // you will get product object
        $_productloader = $this->getLoadProduct($_product->getId()); // for sku
        $_productloader->setCustomattribute($value); // name of your custom attribute
        $_productloader->save();

    }
}